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SubjectRe: [PATCH 0/4] locks: avoid thundering-herd wake-ups
On Wed, Aug 08, 2018 at 11:51:07AM +1000, NeilBrown wrote:
> If you have a many-core machine, and have many threads all wanting to
> briefly lock a give file (udev is known to do this), you can get quite
> poor performance.
>
> When one thread releases a lock, it wakes up all other threads that
> are waiting (classic thundering-herd) - one will get the lock and the
> others go to sleep.
> When you have few cores, this is not very noticeable: by the time the
> 4th or 5th thread gets enough CPU time to try to claim the lock, the
> earlier threads have claimed it, done what was needed, and released.
> With 50+ cores, the contention can easily be measured.
>
> This patchset creates a tree of pending lock request in which siblings
> don't conflict and each lock request does conflict with its parent.
> When a lock is released, only requests which don't conflict with each
> other a woken.

Are you sure you aren't depending on the (incorrect) assumption that "X
blocks Y" is a transitive relation?

OK I should be able to answer that question myself, my patience for
code-reading is at a real low this afternoon....

--b.

>
> Testing shows that lock-acquisitions-per-second is now fairly stable even
> as number of contending process goes to 1000. Without this patch,
> locks-per-second drops off steeply after a few 10s of processes.
>
> There is a small cost to this extra complexity.
> At 20 processes running a particular test on 72 cores, the lock
> acquisitions per second drops from 1.8 million to 1.4 million with
> this patch. For 100 processes, this patch still provides 1.4 million
> while without this patch there are about 700,000.
>
> NeilBrown
>
> ---
>
> NeilBrown (4):
> fs/locks: rename some lists and pointers.
> fs/locks: allow a lock request to block other requests.
> fs/locks: change all *_conflict() functions to return bool.
> fs/locks: create a tree of dependent requests.
>
>
> fs/cifs/file.c | 2 -
> fs/locks.c | 142 +++++++++++++++++++++++++--------------
> include/linux/fs.h | 5 +
> include/trace/events/filelock.h | 16 ++--
> 4 files changed, 103 insertions(+), 62 deletions(-)
>
> --
> Signature

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